s390x/tcg: Implement VECTOR FP CONVERT FROM FIXED 64-BIT

s390x/tcg: Implement VECTOR FP CONVERT FROM FIXED 64-BIT

1. We'll reuse op_vcdg() for similar instructions later, prepare for
   that.
2. We'll reuse vop64_2() later for other instructions.

We have to mangle the erm (effective rounding mode) and the m4 into
the simd_data(), and properly unmangle them again.

Make sure to restore the erm before triggering an exception.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: David Hildenbrand <david@redhat.com>